This, and generally slow loading, seems to be caused by Zandronum forums loading 10 separate CSS files from a single domain, which may kill performance in HTTP 1, as it only loads 4 files in parallel from a single domain.

Given that zandronum.com is fully HTTPS-only, why not just enable HTTP 2 on the web server to speed it up? Then you don't need to merge CSS files to get acceptable performance.